1. Field of the Invention
The present invention relates to an output voltage variable power circuit for supplying a power voltage to a load, particularly to an output voltage variable power circuit which has a plurality of power units and operates them in parallel.
2. Description of the Prior Art
In this type of the output voltage variable power circuit heretofore used, each of plural power units has a reference voltage set by adjusting an output voltage variable resistor. Therefore, the more power units are operated in parallel, the more laborious the adjustment becomes. To solve the problem, as an example in which reference voltages of plural power units can be simultaneously set, a publication of patent application laid-open No. Sho 60-134921 discloses an output voltage variable circuit.
FIG. 7 is a functional block diagram showing a prior-art output voltage variable power circuit disclosed in the publication, which is constituted of a plurality of power units PSU1 to PSUn and a power control device 6. In the power control device 6, a power control portion 61 transmits a power starting signal and a power changing signal to the power units PSU1 to PSUn. The power changing signal is transmitted to the power units PSU1 to PSUn through a signal transmitting circuit 63. A digital signal generating circuit 62 is constituted of a constant-voltage source 62a, a volume 62b which can vary a power from the constant-voltage source 62a to take out an optional voltage, and a digital signal conversion circuit 62c for converting a value of the taken voltage to a digital signal. The digital signal determines the level of the power changing signal.
Since parallel-operating power units PSU1 to PSUn are the same in circuit constitution, the power unit PSU1 representing them is described. A bridge rectifier circuit 71 receives an alternate power from an input power 72, and converts the alternate power to a direct current. A pulse width control circuit 73 receives the power starting signal from the power control portion 61 of the power control device 6 and an error voltage signal from an error detector 74 to drive a drive circuit 75, and controls ON/OFF switching pulse widths of switching elements 76a and 76b. A direct-current intermittent wave which is obtained by switching on or off the switching elements 76a and 76b is transmitted by a transformer 77 to a secondary side, and rectified and smoothed by a rectifier smoothing circuit 78, so that a direct-current power is emitted from between output terminals l and m.
For error detecting resistors 79 and 80, to detect an output voltage between the output terminals l and m, one end of the error detecting resistor 79 is connected to a plus side of the output terminal l and one end of the error detecting resistor 80 is connected to a minus side of the output terminal m. A detecting voltage from a contact of the error detecting resistors 79 and 80 is transmitted to one input terminal of the error detector 74, while a reference voltage which is transmitted from a reference voltage setting circuit 81 and finely adjusted by an output voltage variable resistor 82 is transmitted to the other input terminal of the error detector 74.
An operation is now described using FIG. 7. Since the parallel-operating power units PSU1 to PSUn are the same in circuit constitution, the power unit PSU1 representing them is described.
First, the constant-voltage source 62a of the digital signal generating circuit 62 in the power control device 6 is varied in the volume 62b to take out the voltage, and the value of the taken voltage is converted to the digital signal in the digital signal conversion circuit 62c. The digital signal converted by the digital signal conversion circuit 62c is successively converted to an analog signal via the signal transmitting circuit 63, and transmitted to the reference voltage setting circuit 81 as the power changing signal. For the analog signal transmitted to the reference voltage setting circuit 81, the reference voltage emitted from the reference voltage setting circuit 81 is varied and finely adjusted by the output voltage variable resistor 82 to enter the error detector 74.
The error detector 74 compares the finely adjusted reference voltage with a detecting voltage which is obtained by dividing the output voltage between the output terminals l and m by the error detecting resistors 79 and 80, and transmits an error voltage signal to the pulse width control circuit 73. The pulse width control circuit 73 receives the error voltage signal to drive the drive circuit 75, and controls the ON/OFF pulse widths of the switching elements 76a and 76b, so that the output voltage between the output terminals l and m reaches a normal voltage determined by the reference voltage from the reference voltage setting circuit 81.
As aforementioned, in the power device in which the plural power units PSU1 to PSUn are provided with the power control device 6 in common, one volume 62b provided in the power control device 6 can simultaneously vary the reference voltages of the power units PSU1 to PSUn.
A first problem with the aforementioned prior art lies in that the reference voltage of the power units must be set from the outside. Usually, the power units (PSU) is integrated as a power IC, and the reference voltage setting circuit (81 in FIG. 7) for changing the reference voltage is not included in the IC. Namely, the reference voltage is fixed in the power IC and there is no terminal for changing the reference voltage. In this case, the reference voltage cannot be varied from the outside like the power control circuit in FIG. 7, and thus it is impossible to change the output voltages of the power units in parallel.
A second problem is that when the circuit is sued in, for example, an LCD display bias power or another power which needs to be considered with respect to an influence of an ambient temperature, the output voltage cannot be temperature-compensated.